This invention relates to improvement of a circuitry for adjustment of biasing current for recording, as is incorporated in a two-head type tape-recorder, thus having an eraser head and another head usable dually for recording and reproducing the sound.
In this field of art, it is a conventionally established knowledge that the reproduced sound output level shows variation in accordance with variation in the intensity of the biasing current for the recording, and that correlation between these two variations differs as a function of the frequency of the recorded sound, as diagramatically shown in FIG. 1 of the accompanying drawings, which is a chart with the magnetic tape recording bias current intensity on abscissa and the reproduced sound output level on ordinate, giveing two typical curves with respect to the sounds of 400 Hz and 10 kHz, respectively.
As is apparent, ratio of the reproduced sound output level variation to the recording bias current intensity variation is smaller in the case of comparatively low frequency sound than in the case of comparatively high frequency sound.
Thus, remarkable effect of variation in the intensity of the recording bias current over the frequency characteristic chart of the reproduced sound with a given tape-recording apparatus is seen in the higher frequency range, as illustrated in FIG. 2.
In order to realize optimal frequency characteristics with the widest flat range, it is essential to properly select the recording bias current intensity as will result in the same reproduced sound output level in the lower frequency signal range (as may be represented for instance by 400 Hz) as in the higher frequency signal range (as may be represented for instance by 10 kHz). Such optimal biasing current intensity is designated in FIGS. 1 and 2 at P.sub.o.
Said chart of correlation between the recording bias current intensity and the reproduced sound level also differs or deflects considerably as the magnetic characteristics of the recording tape material differ, and it is also in the higher frequency range that such deflection of the characteristics is prominent.
For instance, FIG. 3 illustrates the comparison of the characteristic curves, similar to those shown in FIG. 1, thus for 400 Hz and 10 kHz, with respect to the tape using CrO.sub.2 as the magnetic material (shown in solid lines) and the tape generally referred to as "normal", using .gamma.-Fe.sub.2 O.sub.3 (in broken lines), and it is thus apparent that the said optimal biasing current intensity P.sub.o ' in the case of CrO.sub.2, to give the same reproduced sound output level both for 400 Hz and 10 kHz, is larger than the intensity P.sub.o of the same optimal nature in the case of .gamma.-Fe.sub.2 O.sub.3. In other words, the former material CrO.sub.2 requires stronger recording bias current for realizing the ideal sound reproduction frequency characteristics, with the widest flat range, than does the latter material .gamma.-Fe.sub.2 O.sub.3.
It is essential, therefore, to select the optimal recording bias current intensity in proper accordance with the magnetic characteristics of the particular tape material actually in use, for realizing the said ideal sound reproduction frequency characteristics with the widest flat range, and in view that one must foresee to use various kinds of tapes, of the materials with magnetic characteristics different from one to another (for instance Cr0.sub.2, FeCr, .gamma.-Fe.sub.2 O.sub.3 and so forth), in tape-recorders, especially those of casette-type, there have by now been in use those wherein the recording bias current is adjustable or variable in accordance with the kind of the magnetic tape actually used.
For instance, there have been tape-recorders with such circuitry for adjusting the recording bias current as may switch over the current intensity in a few pre-set steps or as may effect further precise readjustment by means of continuously manipulating a variable resistor after first selectively switching over the current also in a few steps, in accordance with the kind of the magnetic tape actually used.
As is apparent, it is not possible with the simple former circuitry providing only the shifting-over in a few steps, to always set the truly optimal recording bias current intensity for all the magnetic tapes with the magnetic characteristics different from one to another in a quite wide range. On the other hand, the latter circuitry, providing the continuous precise readjustment after shifting over the recording bias current in a few steps, can principally always set the truly optimal current for any foreseeable kind of magetic tapes, but there still is a difficulty or defect with respect to the mode of manipulation as is required for adjusting the recording bias current, even in this latter type of the circuitry, if the tape-recorder is of the so-called two-head type construction, thus having an eraser head and another head to be used dually for recording and reproducing. Looking into further detail in this respect, it is rather easy in the case of the so-called three-head type tape-recorder, thus having independently workable separate recording and reproducing heads as well as an eraser head, to properly perform the adjustment of the recording bias current, since it is possible in such case to immediately check, by simultaneously reproducing the sound while recording same, the resulting effect of adjusting the recording bias current, but the adjustment procedure is quite troublesome in the case of two-head type tape-recorders, since simultaneous recording and reproducing is impossible in such case and thus repeated alternate procedures of recording and reproducing the sound are here required before reaching the proper adjustment of the recording bias current.
In order to more clearly describe the procedures, reference is made to FIG. 4 which is a block diagram of the essential portion of such two-head type tape-recorder with conventional circuitry for adjustment of the recording bias current.
Designated at 40 is the sound signal line input terminal, 41 is the head to be used dually for recording and reproducing the sound, 42 is a reproducing equalizer amplifier, 43 is a main line amplifier, 44 is a meter amplifier, 45 and 46 are calibration signal generators to give the calibration signals of a lower frequency (for instance 400 Hz) and a higher frequency (for instance 10 kHz), respectively, 47 is a recording amplifier, 48 is a biasing oscilator, 49 is a recording bias current selection switch (with contacts a, b and c destined for instance to CrO.sub.2, FeCr and .gamma.-Fe.sub.2 O.sub.3, respectively), 50 is a variable resistor for adjusting intensity of the recording bias current, 51 is a calibration signal selection switch (with contacts l and h destined to the lower and higher frequency signals, respectively), 52 is a meter to indicate the reproduced sound output level, 53 is a recording and reproduction mode selection switch (with contacts r and p destined to recording and reproducing the sound, respectively) and 54 is the sound signal line output terminal. The symbol +B' represents the power source voltage for the biasing oscilater 48.
As the first step to perform proper adjustment of the recording bias current with the circuitry constructed as above, the recording bias current selection switch 49 is set to a suitable contact position corresponding to the magnetic material of the tape actually used, and the lower frequency calibration signal is recorded. After the recording, this tape-recorder is used in reproduction mode to read and memorize the indication, at the output level meter 52, of the sound reproduced from the said recorded tape. Then, similar recording and reproducing procedures are performed this time with respect to the higher frequency calibration signal. There will normally be seen some difference in the indications of the meter 52 of the lower and higher frequency signals, and in the intention of compensating the unbalance, the recording bias current adjusting variable resistor 50 is manipulated on a trial basis. Upon such trial manipulation, the entire process of recording and reproducing both of the lower and higher frequency calibration signals is repeated, in order to check, by reading the indications of the meter 52, whether the trial manipulation was appropriate or not.
It is thus normally required to cyclically repeat again and again the procedures of: recording the lower frequency calibration signal; reproducing the recorded signal; reading the level indication of the reproduced signal; recording the higher frequency calibration signal; reproducing the recorded signal; checking the level indication balance; and readjusting the variable resistror 50. As is now apparent, the procedures for ultimately obtaining the optimal recording bias current adjustment have the drawbacks of being quite troublesome and of requiring highly skilled operator, and are therefore not very practical.